In electrophotography, an image comprising a pattern of electrostatic potential (also referred to as an electrostatic latent image) is formed on a surface of an electrophotographic element comprising at least an insulative photoconductive layer and an electrically conductive substrate. The electrostatic latent image is usually formed by imagewise radiation-induced discharge of a uniform potential previously formed on the surface. Typically, the electrostatic latent image is then developed into a toner image by contacting the latent image with an electrophotographic developer. If desired, the latent image can be transferred to another surface before development.
In latent image formation the imagewise discharge is brought about by the radiation-induced creation of electron/hole pairs, which are generated by a material (often referred to as a charge-generation material) in the electrophotographic element in response to exposure to the imagewise actinic radiation. Depending upon the polarity of the initially uniform electrostatic potential and the type of materials included in the electrophotographic element, either the holes or the electrons that have been generated migrate toward the charged surface of the element in the exposed areas and thereby cause the imagewise discharge of the initial potential. The remaining non-uniform potential is the electrostatic latent image.
Many electrophotographic elements currently in use are designed to be initially charged with a negative polarity. Such elements contain material which facilitates the migration of positive holes toward the negatively charged surface in imagewise exposed areas in order to cause imagewise discharge. Such material is often referred to as a hole-transport agent. In elements of that type, a positively charged toner material is usually then used to develop the remaining imagewise undischarged areas of negative polarity potential, i.e., the latent image, into a toner image. Because of the wide use of negatively charging elements, considerable numbers and types of positively charging toners have been fashioned and are available for use in electrophotographic developers.
However, for some applications of electrophotography it is more desirable to be able to develop the surface areas of the element that have been imagewise exposed to actinic radiation, rather than those that remain imagewise unexposed. For example, in laser printing of alphanumeric characters it is more desirable to be able to expose the relatively small percentage of surface area that will actually be developed to form visible alphanumeric toner images, rather than waste energy exposing the relatively large percentage of surface area that will constitute undeveloped background portions of the final image. In order to accomplish this while still employing widely available high quality positively charging toners, it is necessary to use an electrophotographic element that is designed to be positively charged. Positive toner can then be used to develop the exposed surface areas, which will have, after exposure and discharge, relatively negative electrostatic potential compared to the unexposed areas, where the initial positive potential will remain. An electrophotographic element designed to be initially positively charged preferably contains an adequate electron-transport agent, that is, a material which facilitates the migration of photogenerated electrons toward the positively charged insulative element surface.
Electrophotographic elements include both those commonly referred to as single layer or single-active-layer elements and those commonly referred to as multiactive, multilayer, or multi-active-layer elements.
Single-active-layer elements are so named because they contain only one layer that is active both to generate and to transport charges in response to exposure to actinic radiation. Such elements typically comprise at least an electrically conductive layer in electrical contact with an active layer. In single-active-layer elements, the active layer contains a charge-generation material to generate electron/hole pairs in response to actinic radiation and an electron-transport and/or hole-transport agent, which comprises one or more of chemical compounds capable of accepting electrons and/or holes generated by the charge-generation material and transporting them through the layer to effect discharge of the initially uniform electrostatic potential. The active layer is electrically insulative except when exposed to actinic radiation, and it sometimes contains an electrically insulative polymeric film-forming binder, which may itself be the charge-generating material, or it may be an additional material that is not charge-generating. In either case, the transport agent(s) is (are) dissolved or dispersed as uniformly as possible in the layer.
Multiactive elements are so named because they contain at least two active layers, at least one charge generation layer (CGL) which is capable of generating charges, i.e., electron/hole pairs, in response to exposure to actinic radiation, and at least one charge transport layer (CTL) which is capable of accepting and transporting charges generated by the charge-generation layer. Such elements typically comprise at least an electrically conductive layer, a CGL, and a CTL. Either the CGL or the CTL is in electrical contact with both the electrically conductive layer and the remaining CTL or CGL. The CGL contains at least a charge-generation material; the CTL contains at least a charge-transport agent; and either or both layers can contain an electrically insulative film-forming polymeric binder.
In multiactive positively charged photoconductor elements of the type employing at least a CGL and a CTL, it is preferable to make the CTL the uppermost layer of the element to protect the more mechanically sensitive CGL from wear. To date, known electron transport agents suffer from problems upon repeated use, such as high dark decay, insufficient electronic charge transport activity, a gradually increasing residual potential or the like. Consequently, the art of photoconductor elements continues to seek new electron transport agents which exhibit sufficient sensitivity, but which do not exhibit disadvantages such as above indicated which might restrict their utilization in positively charged photoconductor elements.
Certain electron transport agents, such as trinitrofluorenone (TNF), which do exhibit a useful level of sensitivity, suffer from the further disadvantage that they are now suspected to be carcinogens.
Cyclic bis-dicarboximide compounds have previously been proposed for use in photoconductor elements. For example, U.S. Pat. Nos. 4,992,349, 4,468,444, and 4,442,193 disclose cyclic bis-dicarboximides having the following structure: ##STR2## where R.sup.1 and R.sup.2, which may be the same or different, represent aryl; or aryl substituted with alkyl, alkoxy, perfluoroalkyl or perfluoralkoxy groups having 2 to 20 carbon atoms; sulfonyl; sulfone; sulfonamide; nitrile; or nitro groups; R.sup.3, R.sup.4, R.sup.5, and R.sup.6, which may be the same or different, represent hydrogen, alkyl having 1 to 4 carbon atoms, alkoxy having 1 to 4 carbon atoms, or halogen; and n is 0 to 3. U.S. Pat. No. 4,992,349 discloses the use of cyclic bis-dicarboximides of Structure A as electron-transport agents. U.S. Pat. Nos. 4,468,444 and 4,442,193 disclose other uses of the cyclic bis-dicarboximides of Structure A in photoconductor elements.
U.S. Pat. No. 5,266,429 discloses photoconductor elements comprising polymers which incorporate cyclic bis-dicarboximide groups of structure A, except that R.sup.1 and R.sup.2 are functional groups linking the cyclic bis-dicarboximide groups to the remainder of the polymer chain. The polymers can be used as binders with monomeric electron-transport agents, or they may be used as polymeric electron-transport agents. When used as polymeric electron-transport agents, they exhibit relatively poor ability to discharge upon exposure to actinic radiation.
EPO Application No. 00 31065 discloses cyclic bis-dicarboximides having the following structure: ##STR3## where R is a saturated or olefinically unsaturated aliphatic or cycloaliphatic residue containing an electron-donor group and each R.sup.1 can independently be a hydrogen, halogen, NO.sub.2, SO.sub.3 H, CN, COOR.sup.2, N(R.sup.2).sub.2 (where R.sup.2 is hydrogen or C.sub.1 -C.sub.4 -alkyl), hydroxyl, or C.sub.1 -C.sub.4 -alkoxy.
Although cyclic bis-dicarboximide compounds have previously been proposed for use as charge transport agents in photoconductor elements, they suffer from one or more of the following drawbacks: insufficient solubility in a binder, or insufficient electron transport ability or increased dark decay. Therefore, there is a need for photoconductor elements using novel cyclic bis-dicarboximide compounds which possess improved characteristics.